Signing up puts 100 credits in your account, once, with no card. For a typical short story that is enough to go from your idea to a storyboard — the story brief, the script, the cast images and the shot plan — or, if you would rather, about 16 images in the image studio. A longer script or a bigger cast can use it up first, and the video itself is bought with credits.
So the honest summary is: the story generator is free to try, the front half of a short drama is covered by the signup grant for a typical short story, and the video is not free. The cheapest clip in the roster costs 108 credits (Grok Imagine Video, 6 seconds), which is more than the grant, and a multi-shot piece runs into the thousands.
The grant is one-time. It is not a monthly allowance and it does not renew. When it runs out you buy credits when you need them — there is no mandatory subscription — and a longer script or a bigger cast can use the grant up before the storyboard, so a very large idea is better started as a short one.